UNITED STATES OF AMERICA DEPARTMENT OF TRANSPORTATION FEDERAL AVIATION ADMINISTRATION RENTON, WASHINGTON 98057-3356

In the matter ofthe petition of Exemption No. 15762

The Boeing Company Regulatory Docket No. FAA-2015-6811
for an exemption from§ 25.341(a) of Title 14, Code ofFederal Regulations

GRANT OF EXEMPTION
By letterdated November 19, 2015, Mr. David Horn, ODALead Administrator, The Boeing Company, 6001 S

. Air Depot, Oklahoma City, Oklahoma, 73135, petitioned the Federal Aviation Administration (FAA) for an exemption from the requirements of§ 25.341(a) ofTitle 14, Code ofFederal Regulations (14 CFR). This exemption, ifgranted, would exemptthe aerial refueling boom and local attachment structure of767-2C tanker airplanes that have been modified by supplemental type certificate (STC) for military operations from the tuned discrete gust criteria of § 25.341(a).
The petitioner requests relieffrom the following regulations:
Section 25.341- Gust and Turbulence Loads (a)- Discrete Gust Design Criteria The petitioner supports its request with the following information:
This section quotes the relevant information from the petitioner's request, with minor edits for clarity. The petitioner quoted the equations and text of§ 25.341(a) in the request; however, we are omittingthat information from this document for brevity. The complete petition is available at the Department ofTransportation's Federal Docket Management System, onthe Internet at http://regulations.gov, in DocketNo

. FAA-2015-6811

.Description ofIssue Application of 14 CFR 25.341(a), Tuned Discrete Gust (TDG) to the STC-modified 767-2C aerial refueling boom (ARB) would result in significantly increased loads, causing impractical redesign ofthe boom and local attachment structure. Aerial refueling aircraft were not considered when tuned discrete gust criteria were originally derived by the Joint Aviation Authority. Tuned discrete gust criteriawere intended for conventional transport aircraft and used conventional transport aircraft accelerations to back out gust velocities using the static gust loads formula. In addition, aerial refueling aircraft were not evaluated for the load effects or economic impact ofthe tuned discrete criteria, as was done with conventional transport aircraft. Tuned discrete gust criteria have not been used for prior boom design on the KC-10A or KC-135. Priorto Amendment 25-86, static gust was the discrete gust requirement in 14 CFR25.341. The replacement ofstatic gust withtuned discrete gust in 14 CFR25.341(a) at Amendment 25-86 was to provide for harmonization ofthe discrete gust requirements with the Joint Aviation Requirements (JARs). Tuned discrete gust was incorporated into the Code ofFederal Regulations so that the requirements would be common and applicants would not have to comply with multiple discrete gust criteria. The change from static gust to tuned discrete gust was for harmonization and was not done with the goal ofincreasing safety as stated in the preamble to Amendment 25-86, "The objective was to achieve common requirements for the certification oftransport airplanes without a substantive change inthe level ofthe safetyprovided by the regulations

." Tuned discrete gust simulates the aircraft being hit with an extreme single discrete gust, with gust velocities backed out from the static gust formula using aircraft center of gravity (CG) accelerations that occur once in 70,000 flight hours

. This gust velocity is then coupled with the worst possible gust frequency. Loads using this type ofcriteria are more significantly affected for boom-type structure than for conventional transport aircraft structure (wing, fuselage, and empennage). Because TDG criteriaderives gust velocities based on aircraft CG accelerations, the accelerations for structure far away from the CG, which create the majority ofthe load, have large variations that are analysisdriven and not validated by fleet data. In addition,the boom structure's constant spanwise mass and stiffness properties cause loads to be more affected by tip accelerations,thus increasingthe loads significantly higherthan past successful design practice. Conventional transport aircraft structure do not have this effect as the distributed mass and stiffness is significantly lower atthe tip and increases towards the root, thus reducing the load effect from the high tip accelerations. As tuned discrete gust criteria were not created for or evaluated on military aircraft and the load effects ofthis criteria are significantly higher for boom-type structure, this criteria is not appropriate for an aerial refueling boom, which is unique military structure. Structural Capability The STC-modified 767-2C boom baseline is the KC-10A. The KC-10A and STCmodified 767-2C booms are structurally similar. They have the same cross-sectional area 2and the same basic materials, with the exception ofsome modem alloys that were implemented on the STC-modified 767-2C. All modem alloys were selected to have equivalent orbetterproperties than the original alloys. The critical frequencies ofthe first natural bending modes ofthe two booms are very similar. The expected usage and gust environment ofthe STC-modified 767-2C and KC-10A booms are very similar. The KC-1OA fleet has an extensive service history with over 34 years and 1.5 million flight hours (59 aircraft). In that time, there have been no incidents ofrecorded boom damage due to turbulence. The STC-modified 767-2C boom and local attachment structure is designed to both the static gust requirements used for the KC-1OA, in addition to USAF-approved military gust requirements. The STC-modified 767-2C boom design provides the same orbetter structural capability as the KC-10A boom. Thus, the STC-modified 767-2C boom design provides sufficient and appropriate structural capability needed for USAF operations. Statement ofPublic Interest Application of 14 CFR25.341(a) at Amendment 25-86 would cause significant modification to the STC-modified 767-2C boom that is utilized only for military missions and has no civil utility. Additionally, the redesign ofthe boom and local attachment structure would be impractical and provide no netbenefit in the configuration and no commensurate increase in safety. The STC-modified 767-2C will be converted to the KC-46A priorto delivery to the USAF. lfthe reliefis not granted, it could lead to delays in the delivery ofthe KC-46A to the USAF at a critical time ofneed. Itremains inthe public's interest to grant Boeing an exemption to 14 CFR 25.341(a) for the STC-modified 767-2C aerial refueling boom and local attachment structure. Statement ofNo Adverse Effect on Safety Existing aircraft with installed booms such as the KC-10A have a long safety history. The KC-1OA was certified in 1981 and the aircraft fleet has 1.5 million flight hours with no gust-related damage affecting the boom. The STC-modified 767-2C and KC-10A booms are structurally very similar. The STC-modified 767-2C boom and local attachment structure is designed to both the KC-1OA static gust criteria as well as the USAF-approved military gust criteria. Thus, applying the later gust criteria of § 25.341(a) for tuned discrete gust does not add additional safety. Conclusion The Boeingposition is that tuned discrete gust criteriain§ 25.341(a) were derived for conventional transport aircraft and are not appropriate for the STC-modified 767-2C aerial refueling boom as the boom is unique military structure. The STC-modified 767- 2C boom's similarity to the KC-10A boom,the safe history ofthe KC-lOA boom, in addition to the fact that the STC-modified 767-2C boom and local attachment structure is designed to both the KC-1OA static gust requirements and the USAF-approved military gust requirements, providesjustification for an exemption from § 25

.341(a)

. Based on 3the rationale above, Boeinghereby requests FAA issuance ofan exemption from the requirements of§ 25.341(a), Amendment 25-86, for the boom and its local attachment structure for the STC-modified 767-2C Model airplane, FAAprojectNo. ST00279MC-T.
Federal Register publication Although the petitionerrequested that action on its petition not be delayed for publication in the Federal Register, the FAAfound that the petition, ifgranted, would set a precedent. Therefore, to allow an opportunity for the public to comment onthe petition, we published a summary ofit in the Federal Register on January 19,2016 (81 FR2941). No comments were received.
The FAA's analysis

This airplane is intended for military aerial refueling without any applicabilityto commercial operations. The design ofthe refueling boom for the modified Model 767-2C airplane is substantially similarto the one used for the Boeing KC-1OA, which has a long and successful service history. The applicant assures the FAA that the boom will comply with USAF-approved gust requirements, and these static gust requirements are at least as stringent as that used for the KC-1 OA. Since the modified 767-2C airplane will perform the same missions as the KC-1OA, this exemption would not adversely impactpublic safety. Itwill enhance the United States military by replacing older, less efficienttanker airplanes and ensure consistency within the current USAF fleet. For this reason, we agree that granting this exemption is in the public interest.

The FAA's decision
In consideration ofthe foregoing, I find that a grant ofexemption is in the public interest. Therefore, pursuantto the authority contained in 49 U.S.C. 40113 and 44701 delegated to me by the Administrator, I grant The Boeing Company an exemption from 14 CFR25.341(a), to the extent necessary to install the aerial refueling boom and local attachment structure on STCmodified 767-2C military tanker airplanes. Operations with the aerial refueling boom installed are limited to military use only, as allowed by FAA Order 8110.101A.

Michael Kaszycki Acting Manager, Transport Airplane Directorate Aircraft Certification Service

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